1. Field of the Invention
The present invention relates to a two-stage crystallization for the continuous production of sugar crystals from syrup, wherein the mother liquor is maintained at supersaturation and crystallization is obtained by crystallizing the syrup in a first stage by subjecting the syrup to heat of evaporation to produce a massecuite, and subsequently effectuating crystallization of the massecuite produced in the first stage by cooling in a second stage. Syrup is constituted by water in which sugar and impurities, which constitute the dry material, are dissolved. In the massecuite, the dry material is constituted by crystals, on the one hand, and the sugar and impurities dissolved in the mother liquor, on the other hand.
2. Description of the Prior Art
In a known process of this type, the first operating stage is effectuated in an apparatus constituted by a tank divided into a succession of crystallizing compartments or cells equipped with heating means, such as steam-fed radiators, which compartments are traversed successively by a massecuite constituted in a first one of the compartments by adding a seed magma to a syrup of dry matter (sugar) and water, and complemented in the succeeding compartments by metered additions of syrup. The additional amounts of syrup fed to each successive compartment and the supply of calories to the massecuite flowing through the compartments are so controlled that the mother liquor is maintained at a state of supersaturation and in a manner permitting the sugar crystals to grow progressively from the inlet to the outlet of the tank, i.e. from the first to the last compartment. The sugar crystallization and the evaporation of water from the mother liquor cause a progressive augmentation of the content of dry material (Brix) in the massecuite and, consequently, increase the viscosity thereof. In this conventional process, this first crystallization stage is so operated that the massecuite has as high a content of sugar crystals as possible and whose viscosity is limited only by the flow properties required for feeding the massecuite to the second crystallization stage in a mixer-cooler. Because of the high viscosity of the massecuite in the last compartment at the outlet of the tank, there is little heat exchange between the massecuite and the heating means in this compartment, and it is necessary to increase the surface of the heating means in this compartment and/or to generate a higher vacuum in the crystallization apparatus.
The second stage of the conventional sugar crystallization is effectuated in a mixer where the massecuite is slowly cooled to advance the crystallization and the growth of the sugar crystals, taking advantage of the reduced solubility of sugar at lower temperatures. Conventional mixers used in the second stage are usually equipped with vanes and thermal exchange elements through which cold water flows. It is also known to use mixers operating under a vacuum, in which cooling is caused by evaporation due to expansion.